Group III-nitride layers with patterned surfaces

ABSTRACT

A fabrication method produces a mechanically patterned layer of group III-nitride. The method includes providing a crystalline substrate and forming a first layer of a first group III-nitride on a planar surface of the substrate. The first layer has a single polarity and also has a pattern of holes or trenches that expose a portion of the substrate. The method includes then, epitaxially growing a second layer of a second group III-nitride over the first layer and the exposed portion of substrate. The first and second group III-nitrides have different alloy compositions. The method also includes subjecting the second layer to an aqueous solution of base to mechanically pattern the second layer.

BACKGROUND

[0001] 1. Field of the Invention

[0002] The invention relates to electrical and optical devices thatincorporate crystalline group III-nitrides.

[0003] 2. Discussion of the Related Art

[0004] Crystalline group III-nitride semiconductors are used in bothelectrical devices and optical devices.

[0005] With respect to electrical devices, group III-nitrides have beenused to make field-emitters. A field-emitter is a conductive structurewith a sharp tip. The sharp tip produces a high electric field inresponse to being charged. The high electric field causes electronemission from the tip. For this reason, an array of field emitters canoperate a phosphor image screen.

[0006] One prior art method has fabricated arrays of field-emitters fromgroup III-nitrides. Group III-nitrides have chemical and mechanicalstability due to the stability of the group III atom-nitrogen bond. Suchstability is very desirable in devices that use an array offield-emitters.

[0007] The prior art method grows the field emitters from groupIII-nitrides. The growth method includes epitaxially growing a galliumnitride (GaN) layer on a sapphire substrate, forming a SiO₂ mask on theGaN layer, and epitaxially growing pyramidal GaN field-emitters incircular windows of the mask. While the growth method producesfield-emitters of uniform size, the field emitters do not have verysharp tips. Sharper tips are desirable to produce higher electronemission rates and lower turn-on voltages.

[0008] With respect to optical devices, group III-nitrides have highrefractive indices. Materials with high refractive indices are desirablein the manufacture of photonic bandgap structures. For a fixed photonicbandgap, such materials enable making a photonic bandgap structure withlarger feature dimensions than would be possible if the structure wasmade from a lower refractive index material.

[0009] One method for making a planar photonic bandgap structureinvolves dry etching a smooth layer of group III-nitride. Unfortunately,the chemical stability of group III-nitrides causes dry etchants to havea low selectivity for the group III-nitride over mask material. For thatreason, a dry etch does not produce a deep surface relief in a layer ofgroup III-nitride. Consequently, the dry-etch method only produces thinplanar photonic bandgap structures from group III-nitrides.

[0010] Unfortunately, light does not efficiently edge couple to thinplanar structures. For this reason, it is desirable to have a methodcapable of fabricating a photonic bandgap structure with a highersurface relief from a group III-nitride.

BRIEF SUMMARY

[0011] Herein a mechanically patterned surface has an array ofdeformations therein, e.g., an array of holes, trenches, or physicallyrough regions.

[0012] Various embodiments provide methods for fabricating layers ofgroup III-nitride with mechanically patterned surfaces. The patternedsurfaces provide functionalities to the resulting structures. Thefabrication methods exploit the susceptibility of nitrogen-polar(N-polar) group III-nitride layers to attack by strong bases. Themethods use basic solutions to wet etch a layer of group III-nitride ina manner that produces a patterned surface. Exemplary patterned surfacesprovide photonic bandgap structures and field-emitter arrays.

[0013] In a first aspect, the invention features a fabrication method.The method includes providing a crystalline substrate and forming afirst layer of a first group III-nitride on a planar surface of thesubstrate. The first layer has a single polarity and also has a patternof holes or trenches that expose a portion of the substrate. The methodincludes epitaxially growing a second layer of a second groupIII-nitride over both the first layer and the exposed portion ofsubstrate. The first and second group III-nitrides have different alloycompositions. The method includes subjecting the second layer to anaqueous solution of base to mechanically pattern the second layer.

[0014] In a second aspect, the invention features an apparatus with amechanically patterned surface. The apparatus includes a crystallinesubstrate with a planar surface and a plurality of pyramidalfield-emitters located over a portion of the surface. The apparatusincludes a layer of a first group-III nitride, which is located onanother portion of the surface, and a layer of a second groupIII-nitride, which is located over the layer of the first groupIII-nitride. The layer of the second group III-nitride is free ofpyramidal surface structures. The field-emitters include the secondgroup III-nitride. The first and second group III-nitrides havedifferent alloy compositions.

[0015] In a third aspect, the invention features an apparatus thatincludes a crystalline substrate and a mechanically patterned layer of afirst group III-nitride that is located on a planar surface of thesubstrate. The apparatus also includes a layer of a second groupIII-nitride that is located on the mechanically patterned layer of thefirst group III-nitride. The layer of second group III-nitride has apattern of columnar holes or trenches therein. The first and secondgroup III-nitrides have different alloy compositions.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1A is a cross-sectional view of a planar structure for afield-emitter array of group III-nitride;

[0017]FIG. 1B is a cross-sectional view of another planar structure fora field-emitter array of group III-nitride field-emitters;

[0018]FIG. 1C shows a flat panel image display that incorporates thefield-emission array of FIG. 1A or 1B;

[0019]FIG. 2 is a cross-sectional view of a structure that incorporatesa layer of group III-nitride that is mechanically periodically patternedwith holes or trenches;

[0020]FIG. 2A is top view of one embodiment of photonic bandgap devicethat incorporates a structure represented by the structure of FIG. 2;

[0021]FIG. 2B is top view of another embodiment of a photonic bandgapdevice that incorporates a structure represented by the structure ofFIG. 2;

[0022]FIG. 2C is top view of another embodiment of a photonic bandgapdevice that incorporates a structure represented by the structure ofFIG. 2;

[0023]FIG. 3 is a flow chart illustrating a method for fabricatingstructures with patterned layers of group III-nitride as shown in FIGS.1A-1B, 2, 2A, and 2B;

[0024]FIG. 4 is oblique view scanning electron micrograph (SEM) of astructure made by an embodiment of the method of FIG. 3 in which the wetetch time is short;

[0025]FIG. 5 is top view SEM of a structure made by an embodiment of themethod of FIG. 3 in which the wet etch time is of intermediate length;

[0026]FIG. 6 is a top view SEM of a structure made by an embodiment ofthe method of FIG. 3 in which the wet etch time is long; and

[0027]FIG. 7 is a flow chart for specific method of fabricating GaNstructures with patterned layers as shown in FIGS. 1A-1B, 2, 2A, and 2B.

[0028] In the Figures and text like reference numbers refer to similarelements.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0029] The chemical stability of the bond between group III metals andnitrogen causes group III-nitride semiconductors to be chemicallyresistant to many etchants. Nevertheless, aqueous solutions of strongbases will etch a nitrogen-polar surface of a layer of groupIII-nitride. Such wet etchants are able to mechanically pattern a groupIII-nitride layer that is already been polarity patterned. Exemplarypatterned surfaces produce field-emitter arrays, as shown in FIGS.1A-1B, and photonic bandgap structures, as shown in FIGS. 2, 2A, and 2B.

[0030]FIG. 1A shows a field-emitter array 10A. The field emitter array10A includes a substrate 12 and a regular pattern of laterallyinter-dispersed columnar first and second regions 14, 15. The first andsecond regions 14, 15 cover a planar surface 17 of the substrate 12. Thesubstrate 12 is a crystalline material such as silicon carbide (SiC) or(0 0 01)-plane sapphire. The first and second regions 15, 16 includerespective (0 0 0 1)-polarity and (0 0 0 {overscore (1)})-polarity formsof a group III-nitride. The group III-nitride of the regions 14, 15 hasa wide lattice mismatch with the planar surface 17 of the substrate 12.

[0031] Herein, the (0 0 0 {overscore (1)})-polarity and (0 0 01)-polarity forms of a group III-nitrides are referred to as the N-polarand metal-polar forms. The N-polar and metal-polar forms have oppositeintrinsic polarizations. The free surfaces of flat N-polar andmetal-polar layers terminate with layers of nitrogen atoms and groupIII-metal atoms, respectively.

[0032] The first and second columnar regions 14, 15 have physicallydifferent surfaces and thus, form a mechanically patterned layer ofgroup III-nitride on the substrate 12. The first regions 14 include oneor more hexagonal pyramids 16 of the group III-nitride. Thus, the firstregions 14 have non-flat exposed surfaces, which include sharp tips 20.The second regions 15 include smooth layers of the group III-nitride andare devoid of pyramidal structures. Thus, exposed surfaces 22 of thesecond regions 15 are smooth and flat. The surfaces 22 of the secondregions 15 are also farther above the planar surface 17 of the substrate12 than the highest tips 20 in the first regions 14.

[0033] The hexagonal pyramids 16 of the first regions 14 have sharpapical points 20 and thus, can function as field emitters. The apicaltips 20 have diameters of less than 100 nanometers (nm). In embodimentswhere the group III-nitride is GaN, the pyramids 16 have tips 20 withdiameters of less than about 20 nm-30 nm and have faces that make anglesof about 56°-58° with the planar surface 17. The pyramids 16 have sixfaces that are (1 0 {overscore (1 1)}) facets.

[0034] Within a single first region 14, the distribution of thehexagonal pyramids 16 and apical tips 20 is random. Various firstregions 14 may have different numbers of the hexagonal pyramids 16. Thesizes of the first regions 14 are constant, because the inter-dispersedsecond regions 15 have the same size and regular lateral distribution.

[0035] In the second regions 15, the layer of first group III-nitriderests on a much thinner base layer 18 that is made of a second groupIII-nitride. The second group III-nitride has a wide lattice mismatchwith the planar surface 17 of the substrate 12. More importantly, thesecond group III-nitride grows with metal-polarity on the surface 17 ofthe substrate 12.

[0036]FIG. 1B shows an alternate embodiment of a field-emitter array10B. The field emitter array 10B includes a substrate 12 and a regularpattern of inter-dispersed columnar first and second regions 14, 15 asalready described with respect to FIG. 1A. In the field-emitter array10B, the second regions 14 include overlapping hexagonal pyramids 16rather than isolated pyramids 16 as in the field-emitter array 10A.Also, the pyramids 16 have a range of sizes and rest on a thick layer 19of N-polar group III-nitride rather than directly on the planar surface17 as in the field-emitter array 10A of FIG. 1A. In the field-emitterarray 10B, the hexagonal pyramids 16 still have sharp apical points 20and thus, can still function effectively as field-emitters. The apicaltips 20 are still lower than the exposed top surfaces 22 of the secondregions 15.

[0037]FIG. 1C shows an embodiment of a flat panel image display 24. Thedisplay 24 incorporates a field-emitter array 10, e.g., array 10A or 10Bfrom FIGS. 1A and 1B. The display 24 also includes metallic electrodes26 and a phosphor screen 28. The metallic electrodes 26 are supported bythe flat top surface 22 of the field emitter array's second regions 15.The top surfaces 22 support the metallic electrodes 26 along a planethat is nearer to the phosphor screen 28 than are the tips 20themselves. For that reason, the metallic electrodes 26 are able tocontrol emission of electrons from the field-emitter array. The metallicelectrodes 26 function as control gates for field-emitters in adjacentfirst regions 14. Supporting the metallic electrodes 26 on the secondregions 22 conveniently avoids a need to self-align the electrodes onindividual tips 20. Such an alignment process would be complex, becausethe positions of the tips 20 are random in individual first regions 14.

[0038]FIG. 2 shows another structure 30 that has a mechanicallypatterned layer 32 of a first group III-nitride. The layer 32 is locatedon a planar surface 17 of crystalline substrate 12 e.g., the (0 0 01)-plane of a sapphire substrate. The layer 32 includes a regular arrayof identical columnar holes or trenches 34. The holes or trenches 34have substantially rectangular cross sections and traverse the entirethickness of layer 32. The layer 32 rests on a mechanically patternedbase layer 18. The base layer 18 is a second crystalline groupIII-nitride with a different alloy composition than the first groupIII-nitride. The base layer 18 aligns epitaxially on the planar surface17 to be group III metal-polar.

[0039] For the pair of layers 18 and 32, exemplary pairs of second andfirst group III-nitride semiconductors are: the pair AlN and GaN or thepair AlN and AlGaN.

[0040] The layer 32 has thickness that is typically 100-10,000 timesthan the thickness of the base layer 18. An exemplary GaN layer 32 has athickness of 30 μm or more, and an exemplary AlN base layer 18 has athickness of only about 20 nm-30 nm. The base layer 18 only has to bethick enough to align the polarization of another layer located on thebase layer 18.

[0041] In optical devices, the layer 32 usually functions as an opticalcore of a planar waveguide. The waveguide receives input light 36 via anedge 37 and transmits output light 38 via an opposite edge 39. Such edgecoupling of the layer 32 to optical fibers and other optical waveguidesis more efficient for embodiments in which the layer 32 is thicker. Itis thus, advantageous that the layer 32 can be relatively thick, i.e.,30 μm or more, because such a thicker layer 32 enables efficient endcoupling to standard optical fibers and waveguides.

[0042] The patterned thick layer 32 can, e.g., be a thick photonicbandgap structure. Thick photonic bandgap structures provide moreefficient optical edge coupling than thinner photonic bandgap structuresthat can be made by dry etching.

[0043]FIGS. 2A and 2B show two exemplary planar photonic bandgapstructures 30A, 30B. Cross-sectional views through the structures 30Aand 30B are faithfully represented in FIG. 2. The structures 30A and 30Binclude a layer 32 of a metal-polar group III-nitride, i.e., (0 0 01)-plane group III-nitride. The layer 32 is located on the top surfaceof the crystalline substrate 12 shown in FIG. 2. The layer 32 ismechanically patterned by an array of substantially identical columnarfeatures 34A, 34B. The columnar features 34A, 34B are holes and trenchesin the structures 30A and 30B, respectively.

[0044] The holes 34A and trenches 34B form regular arrays that have oneand two discrete lattice symmetries, respectively. For this reason, theholes 34A and trenches 34B produce respective 2-dimensional and1-dimensional periodic modulations of the refractive index of the layer32. The refractive index modulations produce a photonic bandgapstructure for selected lattice lengths in the arrays. Lattice lengthsthat are odd integral multiples of ¼ times the effective wavelength ofinput light in the medium will produce photonic bandgap structures.

[0045]FIG. 2C shows a photonic bandgap structure 30C similar to thephotonic bandgap structure 30A of FIG. 2A except that the holes and thegroup III-nitride material layer are exchanged. In the structure 30C,the layer 32C of group III-nitride is a two-dimensional array ofisolated pillars. Between the group III-nitride pillars is aninterconnected two-dimensional pattern of trenches 34C. The trenches 34Cisolate the pillars from each other.

[0046]FIG. 3 illustrates a method 40 for fabricating a structure with amechanically patterned layer of group III-nitride, e.g., as shown inFIGS. 1A-1B, 2, 2A, or 2B.

[0047] The method 40 includes forming a metal-polarity first layer of afirst group III-nitride on a selected planar surface of a crystallinesubstrate (step 42). Forming the layer includes performing an epitaxialgrowth of a first group III-nitride, and mechanically patterning thelayer lithographically. The composition of the first group III-nitrideis selected to insure that the epitaxial growth produces ametal-polarity layer. The mechanical patterning produces a regularpattern of identical holes or trenches that expose a portion of thesubstrate through the layer.

[0048] Next, the method 40 includes epitaxially growing a thicker secondlayer of a second group III-nitride over the first layer and the exposedportion of the substrate (step 44). Over the first layer, the secondlayer grows with metal-polarity. Over the exposed portion of thesubstrate, the second layer grows with N-polarity. The first and secondgroup III-nitrides have different alloy compositions, e.g., AlN and GaN,and have a wide lattice-mismatch with the substrate.

[0049] Finally, the method 40 also includes subjecting the second layerto an aqueous solution of a strong base such as potassium hydroxide(KOH) or sodium hydroxide (NaOH) (step 46). The aqueous solutionmechanically patterns the second layer by selectively etching N-polarsurfaces. Aqueous solutions of strong bases do not significantly etchmetal-polar surfaces of group III-nitrides. The form of the mechanicalpatterning qualitatively depends on the etching time and theconcentration of the etchant.

[0050]FIG. 4 is a scanning electron micrograph (SEM) of a polaritystriped GaN layer 50 that has been wet etched with a 2 molar aqueoussolution of KOH for 45 minutes. The GaN layer was maintained at atemperature of about 90° C. during the wet etch.

[0051] The etched GaN layer 50 has N-polar GaN stripes 52 and Ga-polarGaN stripes 54. The relatively short etch has removed significantmaterial from the N-polar GaN stripes 52 without removing significantmaterial from the Ga-polar GaN stripes 54. In the N-polar stripes 52,the etch produces a surface formed of densely packed hexagonal GaNpyramids, e.g., as shown in FIG. 1B. The pyramids have various sizes andsharp apical tips with diameters of about 20-30 nm or less.

[0052] Measurements indicate that the pyramid density, ρ_(Δ), varieswith etching temperature, T, as: [ρ_(Δ)]⁻¹=[ρ_(Δ0)]⁻¹exp(−E_(a)/k_(B)T)where k_(B) is Boltzmann's constant. For a 2 molar solution of KOH, a 15minute etch, and temperatures between 25° C. and 100° C., measurementsshow that the activation energy E_(a) equal to about 0.587 eV.

[0053] The inventors believe that the wet KOH etch produces adistribution of packed hexagonal GaN pyramids, in part, due to theGa-polar GaN stripes 54 that are not etched. In particular, the Ga-polarstripes laterally confine the N-polar stripes 52 so that the etchantattacks top surface of the N-polar stripes 52 rather than side surfacesthereof. The KOH wet etch produces a dense-packing of sharp tippedhexagonal GaN pyramids when the N-polar GaN stripes 52 have widths ofabout 7 microns (μm). It is believed that a dense packing of hexagonalpyramids will also result from a wet KOH etch of GaN surfaces in whichN-polar GaN stripes have widths of about 100 μm or less. It is nothowever, believed that a wet KOH etch of an unconfined planar surfaceN-polar GaN will produce a dense packing of sharp tipped, hexagonal GaNpyramids.

[0054]FIG. 5 is an SEM image showing a polarity-striped GaN layer 50that has been wet etched with a 4 molar aqueous solution of KOH for 60minutes. Again, the GaN layer was maintained at a temperature of about90° C. during the wet etch.

[0055] The more intense etch has removed all material from the N-polarGaN stripes 52 except for isolated hexagonal GaN pyramids 56. The wetetch stopped on the underlying crystalline sapphire substrate. Thisintermediate length etch produces patterning like that of FIG. 1A, atleast, within individual N-polar GaN stripes 52. Within these regions,the hexagonal GaN pyramids 56 have a random distribution.

[0056]FIG. 6 is an SEM image of a polarity-striped GaN layer 50 that hasbeen etched with a 4 molar aqueous solution of KOH for more than 60minutes. Again, the GaN layer is maintained at a temperature of 25°C.-125° C. and preferably of about 90° C. during the wet etch.

[0057] This longer etch has completely removed the original N-polar GaNstripes 52. As a result, substantially vertical trenches separate theunetched Ga-polar stripes 54. The sidewalls of the Ga-polar stripes 54are not completely vertical, because the wet etchant slowly attackssidewalls of Ga-polar GaN layers. Aqueous solutions with higherconcentrations of KOH than 4 molar tend to erode exposed side and endsurfaces of Ga-polar stripes 54. The resulting structure has a patternedGa-polar layer of group III-nitride like structures 30, 30A, and 30B ofFIGS. 2, 2A, and 2B.

[0058]FIG. 7 illustrates a method 60 for fabricating GaN structures thatare mechanically patterned as in FIGS. 1A, 1B, 2, 2A, and 2B. The method60 includes preparing a planar sapphire growth substrate (step 62),growing and patterning a Ga-polarity aligning layer on the substrate(step 64), and epitaxially growing a polarity-patterned GaN layer overthe aligning layer (step 66). The method 60 also includes wet etchingthe GaN layer to produce mechanical patterning by selectively removingGaN in the N-phase regions (step 68).

[0059] In step 62, preparing the sapphire growth substrate includescleaning a (0 0 0 1)-plane surface of a crystalline sapphire substrate.The cleaning includes washing the surface for 1 minute in an aqueouscleaning solution. Mixing a first aqueous solution having about 96weight % H₂SO₄ with a second aqueous solution having about 30 weight %H₂O₂ produces the aqueous cleaning solution. During the mixing, about 10volume parts of the first solution are combined with one volume part ofthe second solution. The cleaning also includes rinsing the washedsurface with de-ionized water and then spin-drying the sapphire growthsubstrate.

[0060] In step 62, preparing the growth substrate also includesdegassing the sapphire substrate in the buffer chamber of a molecularbeam epitaxy (MBE) system at about 200° C. The degassing continues untilthe chamber pressure is below about 5×10⁻⁹ Torr. After the degassing,the sapphire substrate is transferred to the growth chamber of theplasma-assisted MBE system.

[0061] In step 64, growing and patterning a Ga-polarity aligning layerincludes performing an MBE growth of an AlN layer on the sapphiresubstrate (substep 64 a). To perform the MBE growth, the temperature ofthe growth chamber is raised at a rate of about 8° C. per minute to afinal temperature of about 720° C. The sapphire substrate is maintainedat a uniform temperature with the aid of a 300 nm thick layer oftitanium deposited on the substrate's back surface.

[0062] The MBE system grows the AlN layer to a thickness of about 20 nmto 30 nm. This thin AlN layer is sufficiently thick to cover the entireexposed surface of the sapphire substrate. In the model 32P MolecularBeam Epitaxy system made by Riber Corporation of 133 boulevard National,Boite Postale 231, 92503 Rueil Malmaison France, the growth conditionsare: Al effusion cell temperature of about 1050° C., nitrogen flow rateof about 2 sccm, and RF power of about 500 watts (W).

[0063] In step 64, forming the patterned AlN layer 12 includesperforming an MBE growth of about 50 nm of protective GaN on the alreadygrown AlN layer (substep 64 b). The GaN layer protects the underlyingAlN from oxidation during subsequent removal of the substrate from theMBE growth chamber. Growth conditions for the GaN layer are similar tothose for the MBE growth of the AlN layer except that the temperature israised in the Ga effusion cell rather than in the Al effusion cell.During this growth, the Ga effusion cell has a temperature of about1000° C. to about 1020° C.

[0064] After cooling the sapphire substrate to about 200° C., theGaN/AlN layer is lithographically patterned with a regular array ofwindows that expose selected portions of the sapphire substrate (substep64 c). The patterning step includes forming a photoresist mask on theGaN layer and then, performing a conventional chlorine-based plasma etchto remove unmasked portions of the GaN/AlN layer. Exemplary conditionsfor the plasma etch are: RF source power of about 300-500 watts, sourcebias of 100 volts to 200 volts, chlorine-argon flow rate of about 10-25sccm (20% to 50% of the flow being argon), and a gas pressure of about 1millitorr to about 10 millitorr. The plasma etch produces a preselectedpattern of GaN topped AlN regions.

[0065] After the plasma etch, the sapphire substrate with a pattern ofGaN topped AlN regions is cleaned in an aqueous solution of HCl, rinsedin de-ionized water, and blown dry with nitrogen. This aqueous cleaningsolution includes between about 36.5 weight % HCl and about 48 weight %HCl. Then, the above-described steps are again used to reintroduce thesapphire substrate into the MBE system.

[0066] In step 66, epitaxially growing a GaN layer includes performing aplasma enhanced MBE growth of a GaN layer to a thickness of about 2 μmor more. During the MBE growth, the system conditions, are: Ga effusioncell temperature of about 1000° C. to about 1020° C., nitrogen flow rateof about 2 sccm, and RF power of about 500 watts (W). During thisgrowth, the GaN topped AlN regions initiate growth of Ga-polar GaN, andthe exposed regions of the sapphire substrate 10 initiate growth ofN-polar GaN.

[0067] In step 68, the anisotropic wet etching includes immersing theGaN layer and substrate in an aqueous solution of KOH. Exemplary wetetches use 1 to 4 molar aqueous solutions of KOH and etch periods ofabout 15 minutes to 60 minutes at temperatures of 100° C. Theconcentration of KOH and etch time determines the qualitative form ofthe resulting mechanical patterning as illustrated in FIGS. 4-6. The wetetch selectively removes GaN with N-polarity. Nevertheless, wet etcheswith more basic aqueous solutions than 4 molar KOH can erode end facesof Ga-polar portions of the original GaN layer.

[0068] From the disclosure, drawings, and claims, other embodiments ofthe invention will be apparent to those skilled in the art.

What we claim is:
 1. A method, comprising: providing a crystallinesubstrate with a planar surface; forming a first layer of a first groupIII-nitride on the planar surface, the first layer having a singlepolarity and having a pattern of holes or trenches, that expose aportion of the substrate; then, epitaxially growing a second layer of asecond group III-nitride over the first layer and the exposed portion ofsubstrate, the first and second group III-nitrides having differentalloy compositions; and subjecting the second layer to an aqueoussolution of base to mechanically pattern the second layer.
 2. The methodof claim 1, wherein the single polarity is group III metal-polarity. 3.The method of claim 2, wherein one of the group III-nitrides comprisesaluminum and the other of the group III-nitride comprises gallium. 4.The method of claim 2, wherein the act of epitaxially growing producesone polarity of the second group III-nitride over the first groupIII-nitride and an opposite polarity of the second group III-nitrideover the exposed portion of substrate.
 5. The method of claim 2, whereinthe second layer consists of first and second regions, the first regionbeing located over the exposed portion of the substrate; and wherein thesubjecting produces pyramids in one of the first and second regions andproduces a smooth layer in the other of the first and second regions. 6.The method of claim 5, wherein the first group III-nitride is disposedbetween the planar surface and the second regions.
 7. The method ofclaim 2, wherein the subjecting produces an array of columnar holes ortrenches in the second layer of a second group III-nitride.
 8. Themethod of claim 7, wherein at least some of the columnar holes ortrenches uncover a portion of the planar surface of the substrate. 9.The method of claim 7, wherein the array forms a pattern with at leastone discrete lattice symmetry.
 10. The method of claim 1, wherein thesingle polarity is metal-polarity.
 11. The method of claim 10, whereinthe act of epitaxially growing produces one polarity of the second groupIII-nitride over the first group III-nitride and an opposite polarity ofthe second group III-nitride over the exposed portion of substrate. 12.The method of claim 10, wherein the second layer consists of first andsecond regions, the first region being located over the exposed portionof the substrate; and wherein the subjecting produces pyramids in one ofthe first and second regions and produces a smooth layer in the other ofthe first and second regions.
 13. An apparatus, comprising: acrystalline substrate with a planar surface; and a plurality ofpyramidal field-emitters located over a portion the surface; a layer ofa first group III-nitride located on another portion of the surface, thefield-emitters comprising a second group III-nitride; a layer of thesecond group-III nitride semiconductor being over the layer of a firstgroup III-nitride and being free of pyramidal surface structures; andwherein the first and second group III-nitrides have different alloycompositions.
 14. The apparatus of claim 13, wherein the first groupIII-nitride comprises aluminum and the second group III-nitridecomprises gallium.
 15. The apparatus of claim 13, further comprising: aphosphor screen located to receive electrons emitted by the pyramidalfield-emitters; and a plurality of gate electrodes located between thelayer of the second group III-nitride and the phosphor screen.
 16. Theapparatus of claim 15, wherein the gate electrodes being mechanicallysupported by the layer of the second group III-nitride.
 17. Anapparatus, comprising: a crystalline substrate having a planar surface;a mechanically patterned layer of a first group III-nitride beinglocated on the planar surface; and a layer of a second group III-nitridebeing located on the mechanically patterned layer of a first groupIII-nitride and having a pattern of columnar holes or trenches therein;and wherein the first and second group III-nitrides have different alloycompositions.
 18. The apparatus of claim 17, wherein bottom surfaces ofat least some of the holes or trenches contact the planar surface. 19.The apparatus of claim 18, wherein the layer of a second groupIII-nitride comprising a regular two-dimensional array of pillars. 20.The apparatus of claim 17, wherein the second group III-nitridecomprises gallium and the first group III-nitride comprises aluminum.